U.S. patent number 11,141,278 [Application Number 16/497,758] was granted by the patent office on 2021-10-12 for surgical implants comprising graded porous structures.
This patent grant is currently assigned to VITO NV. The grantee listed for this patent is VITO NV. Invention is credited to Simge Danaci, Jasper Lefevere, Steven Mullens, Lidia Protasova, Dirk Vangeneugden.
United States Patent |
11,141,278 |
Mullens , et al. |
October 12, 2021 |
Surgical implants comprising graded porous structures
Abstract
A surgical implant may include a porous structure with
interconnected pores for ingrowth of bone into the porous
structure. The porous structure has an arrangement of fibres which
are attached to one another, the fibres being arranged in stacked
layers. The porous structure has a surface including different
regions having different porosities. A method of making the above
surgical implant is also described.
Inventors: |
Mullens; Steven (Mol,
BE), Protasova; Lidia (Mol, BE), Danaci;
Simge (Grenoble, FR), Vangeneugden; Dirk (Mol,
BE), Lefevere; Jasper (Mol, BE) |
Applicant: |
Name |
City |
State |
Country |
Type |
VITO NV |
Mol |
N/A |
BE |
|
|
Assignee: |
VITO NV (Mol,
BE)
|
Family
ID: |
1000005861662 |
Appl.
No.: |
16/497,758 |
Filed: |
March 29, 2018 |
PCT
Filed: |
March 29, 2018 |
PCT No.: |
PCT/EP2018/058245 |
371(c)(1),(2),(4) Date: |
September 25, 2019 |
PCT
Pub. No.: |
WO2018/178313 |
PCT
Pub. Date: |
October 04, 2018 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20200030102 A1 |
Jan 30, 2020 |
|
Foreign Application Priority Data
|
|
|
|
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Apr 13, 2017 [EP] |
|
|
17166453 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61F
2/30907 (20130101); A61F 2002/30915 (20130101); A61F
2002/30985 (20130101); A61F 2002/30028 (20130101); A61F
2002/30914 (20130101); A61F 2002/30011 (20130101); A61F
2002/348 (20130101); A61F 2002/30006 (20130101); A61F
2002/30769 (20130101); A61F 2002/30971 (20130101) |
Current International
Class: |
A61F
2/30 (20060101); A61F 2/34 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1135321 |
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Nov 1996 |
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CN |
|
1665551 |
|
Sep 2003 |
|
CN |
|
1446109 |
|
Oct 2003 |
|
CN |
|
201631426 |
|
Nov 2010 |
|
CN |
|
0189546 |
|
Aug 1986 |
|
EP |
|
1820475 |
|
Aug 2007 |
|
EP |
|
Other References
Jul. 9, 2018, European Patent Office, International Search Report
and Written Opinion in PCT/EP2018/058245, which is the
International Application to this U.S. Application. cited by
applicant.
|
Primary Examiner: Sharma; Yashita
Attorney, Agent or Firm: Kolitch Romano LLP
Claims
The invention claimed is:
1. A surgical implant, comprising: a porous structure having
interconnected pores configured for ingrowth of bone into the
porous structure, wherein the porous structure comprises an
arrangement of fibers which are attached to one another, wherein
the fibers are arranged in layers, and wherein the layers are
stacked; and wherein the porous structure includes a surface
including a plurality of regions having different porosities, the
different porosities being determined by the arrangement of fibers;
and wherein the porous structure is attached to a dense part, and
wherein the surface is disposed opposite the dense part, such that
each of the regions having different porosities extend from the
surface toward the dense part.
2. The surgical implant of claim 1, wherein the dense part includes
an interface with the porous structure, wherein a direction of
stacking of the layers of the porous structure is oriented along a
direction of approach to the interface or a direction away from the
interface.
3. The surgical implant of claim 1, wherein the surface has a first
porosity in a first region of the surface and a second porosity in
a second region of the surface, a difference between the first
porosity and the second porosity being at least 4%.
4. The surgical implant of claim 3, wherein the difference between
the first porosity and the second porosity is at least 6%.
5. The surgical implant of claim 3, wherein the first porosity is
between 45% and 90% and wherein the second porosity is between 40%
and 85%.
6. The surgical implant of claim 5, wherein the first porosity is
between 70% and 90% and wherein the second porosity is between 40%
and 60%.
7. The surgical implant of claim 1, wherein the porous structure
has an average porosity between 50% and 80%.
8. The surgical implant of claim 1, wherein the fibers each have a
diameter between 20 .mu.m and 5 mm.
9. The surgical implant of claim 1, wherein fibers of consecutive
layers interpenetrate, wherein a ratio between a penetration depth
between the fibers of consecutive layers and a diameter of the
fibers is between 0.05 and 0.5.
10. The surgical implant of claim 9, wherein the ratio is between
0.1 and 0.5.
11. The surgical implant of claim 1, wherein a spacing between
adjacent fibers of a same layer is between 10 .mu.m and 5 mm.
12. The surgical implant of claim 11, wherein the spacing between
the fibers in at least one layer changes between a first region and
a second region to obtain the different porosities.
13. The surgical implant of claim 1, wherein the porous structure
comprises a porosity gradient in a direction orthogonal to the
surface.
14. The surgical implant of claim 1, wherein the porous structure
comprises a porosity gradient wherein the porosity decreases in a
direction of approach to the dense part.
15. The surgical implant of claim 13, wherein a penetration depth
between the fibers of consecutive layers changes along a direction
of the porosity gradient.
16. The surgical implant of claim 1, wherein the fibers comprise
micropores.
17. The surgical implant of claim 1, wherein the different regions
having different porosities alternate on the surface.
18. The surgical implant of claim 1, wherein each of the regions
having different porosities extend from the surface to the dense
part through a depth of the porous structure.
Description
TECHNICAL FIELD
The present disclosure is related to surgical implants comprising a
scaffold structure for bone ingrowth. In particular, the present
disclosure is related to surgical implants of the above kind in
which the scaffold structure comprises a graded porosity.
INTRODUCTION
From the review article "Graded/Gradient Porous Biomaterials," by
Xigeng Miao and Dan Sun, Materials 2010, 3, 26-47 it is known to
use graded porous implants for repairing bone-cartilage complex
tissue. The part with larger pore size is implanted into bone for
bone ingrowth, whereas the part with smaller pore size is to allow
cartilage to grow in. In other words, the graded porous implant can
be used to select or promote attachment of specific cell types on
and in the implant prior to and/or after implantation. The part for
bone ingrowth and the part for cartilage ingrowth can be made of
different materials. The gradient of material properties may range
from one which is suitable for load bearing to one which is
suitable for soft tissue regeneration.
U.S. Pat. No. 4,978,355 describes a metal grid embedded in the
contact surface of a plastic implant. An additional anchoring
surface for ingress of bone tissue is secured to the embedded grid.
The anchoring surface is formed of layers of metal wire which are
stacked and secured by sintering.
US 2005/0112397 describes a porous structure having a plurality of
stacked bonded sheets. The sheets have a plurality of at least
partially overlapping apertures formed therein, produced by
perforation. Perforating the sheets to create the apertures allows
for obtaining differential porosity within the sheet or from sheet
to sheet. Regions of high porosity are separated by regions of
lower porosity.
Research has indicated that different levels of porosity and pore
size of the scaffold structure have an impact on the amount of bone
ingrowth and the mechanical stability of the implant. Dense
scaffold structures have good mechanical properties but poor bone
ingrowth properties. On the contrary, more porous structures
provide good biological performance but have a low mechanical
strength. The rate of tissue ingrowth in the porous structure is
also dependent on the availability of a large surface area for
cells to attach and grow on. It is known that most bone forming
cells grow on a substratum surface rather than grow in a suspended
manner in the cell culture medium. In this regard, a large pore
surface area means that a large bone-material interfacial bonding
area can be provided.
Furthermore, interconnected porosity promotes the organisation of
vascular canals that can ensure the supply of blood and nutrients
for the viability of bone.
SUMMARY
Osseointegration is important for many surgical implants, however
it is not easily stimulated and/or controlled. Bone ingrowth and
vascularisation strongly depend on macroporosity parameters such as
pore size, pore size distribution and pore interconnectivity. In
order to optimize mechanical properties and macroporosity, several
graded/gradient implants materials have been proposed, particularly
using additive manufacturing technologies.
Despite the advances to date, there is still a need in the art of
improved scaffold structures for surgical implants. In particular,
there is a need of providing such scaffold structures which enhance
the promotion of bone ingrowth into the structure, yet allowing
sufficient freedom in designing the scaffold structure for optimal
mechanical properties. There is a need of providing such implants
having improved tissue anchoring capabilities. There is also a need
in the art of manufacturing scaffold structures of the above kind
in a cost-effective way.
According to a first aspect of the disclosure, there is therefore
provided a surgical implant as set out in the appended claims. The
surgical implant comprises a porous structure with interconnected
pores. The pores have sizes suitable for ingrowth of bone and/or
soft tissue into the porous structure. The porous structure
comprises an arrangement of fibres which are attached to one
another and are arranged in advantageously planar layers, the
layers being stacked. According to aspects of the disclosure, the
porous structure comprises a surface comprising different regions
having different porosities. Advantageously, the arrangement of
fibres extends to the surface and determines the different
porosities by different arrangements of the fibres in the different
regions. Advantageously, the different porosities are determined by
(different) interspaces between adjacent or consecutive fibres. The
term interspace can but does not necessarily refer to the
inter-fibre distance. Rather, the term refers more generally to
(the size of) the interstitial voids delimited by fibres. Different
parameters may influence the porosity of the arrangement of fibres,
such as fibre diameter, inter-fibre distance, stacking factor,
fibre orientation, etc.
According to a second aspect of the disclosure, there is provided a
method of manufacturing a surgical implant of the above kind as set
out in the appended claims. The method comprises the steps of
forming fibres in advantageously planar layers which are stacked on
top of one another, and connecting the fibres of consecutive layers
to one another to obtain a porous structure, e.g. a network of
fibres. According to aspects of the disclosure, the method
comprises the step of arranging the fibres in proximity of a
surface of the porous structure such that the surface comprises
different regions having different porosities. Advantageously, the
fibres are arranged with different interspaces in different regions
of the surface.
BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the disclosure will now be described in detail with
reference to the appended drawings, wherein same reference numerals
illustrate same features.
FIG. 1 represents a cross sectional view of an example surgical
implant according to aspects of the disclosure invention.
FIG. 2 represents a unit pore cell as defined in an orthogonal
fibre disposition scheme, with fibres of consecutive layers being
orthogonal to one another.
FIG. 3 represents a cross sectional view of an example surgical
implant according to an alternative aspect of the disclosure,
wherein different regions have different pore interconnectivity in
a direction of build of the porous structure.
FIG. 4 represents a cross sectional view of an example surgical
implant as in FIG. 1, in which additionally a porosity gradient is
applied in a direction of build of the layers by changing the
inter-fibre distance.
FIG. 5 represents a cross sectional view of an example surgical
implant as in FIG. 4, in which additionally the stacking factor is
changed through the layers in the direction of the porosity
gradient.
FIG. 6 represents a cross sectional view of an example surgical
implant which differs from the implant of FIG. 4 in that the
porosity gradient is obtained by a change of fibre diameter between
layers.
FIG. 7 represents a cross sectional view of an example surgical
implant which differs from the implant of FIG. 4 in that the fibres
are microporous.
FIG. 8 represents an example fibre arrangement pattern as seen from
a direction perpendicular to the plane of the layers in which the
fibres are arranged.
DETAILED DESCRIPTION
For purposes of illustration, aspects of the disclosure will be
described in relation to a particular example of an acetabular
component of a hip implant. It will however be convenient to note
that indicated aspects are readily applicable to other kinds of
implants, such as spinal implants, cranial implants, maxillofacial
implants and dental implants.
FIG. 1 schematically depicts an implant 10 according to aspects of
the disclosure. The implant 10 is shown as an acetabular component
of a hip prosthesis and comprises a scaffold part 11. In this
particular example, the scaffold part 11 is attached to a dense
part 12. The dense part 12 forms a shell of hemispherical shape
which may be provided with a liner forming a receiving part of a
ball joint. A femoral component of the hip prosthesis (not shown)
typically comprises a ball which is accepted in the liner. It will
be convenient to note that in other types of implants, the dense
part may be omitted.
The scaffold part 11 is a porous structure having interconnected
pores which are configured for ingrowth of bone. The scaffold part
11 comprises an external surface 111 forming an interface with the
surrounding sound bone structure. Advantageously, the external
surface 111 is located opposite surface 115 which forms an
interface of attachment to the dense part 12. According to an
aspect of the present disclosure, the surface 111 comprises
different regions having different porosities. By way of example,
the surface 111 comprises first regions 112 and second regions 113.
The first regions 112 have a higher volume porosity (determined in
a volume contiguous to the surface 111) compared to the second
regions 113. In other words, the second regions 113 will have
higher density compared to the first regions 112.
According to an aspect, porosity and/or pore size of the first
regions 112 and of the second regions 113 can be selected such that
the first regions of higher porosity will promote bone ingrowth,
while the second regions of lower porosity will promote the ingress
of vascular canals into the scaffold structure 11. These vascular
canals provide for transport paths for supply of nutrients and
cells which further promote bone formation. Therefore, providing on
the external surface 111 adjacent regions of higher porosity and
lower porosity will provide for parallel paths for ingrowth of bone
and vascular canals, and as a result, will enhance the speed with
which bone will grow into the scaffold structure 11.
Advantageously, a plurality of the second regions 113 are provided
adjacent or in between a plurality of the first regions 112.
Advantageously, the first regions 112 and the second regions 113
alternate on the surface 111. The area of extension of the first
regions and of the second regions is not particularly limited.
Advantageously the first regions and the second regions each extend
over an area of at least 4 mm.sup.2, advantageously at least 5
mm.sup.2, advantageously at least 10 mm.sup.2, advantageously at
least 20 mm.sup.2.
Advantageously, these first and second regions 112, 113 can further
extend into the depth of the scaffold structure 11, e.g. until
surface 115.
According to an aspect, such a structure 11 with alternating
regions of higher porosity and regions of lower porosity is
obtained by forming the scaffold structure 11 out of an arrangement
of fibres 13, 14. The fibres 13, 14 are disposed in layers 151,
152, 153, 154, etc which are stacked on top of one another, and
which are advantageously parallel to one another. This arrangement
of fibres extends to the surface 111. Fibres of consecutive layers
are attached to one another, and thereby advantageously form a
construct which is monolithic, being the scaffold structure 11.
Such monolithic and porous structures can be obtained by well-known
additive manufacturing techniques, such as three-dimensional fibre
deposition, three-dimensional powder deposition or similar solid
free-form fabrication techniques. The fibres or filaments can be
extruded as a paste from a nozzle, as is the case with 3D fibre
deposition, or can be 3D printed starting from a powder layer which
can e.g. be selectively melted (selective laser sintering), or
selectively bound with an adhesive (3D printing).
3D fibre deposition (3DFD) (also called robocasting) comprises the
extrusion of an advantageously highly viscous paste loaded with
metallic or ceramic particles through a thin nozzle. In this case,
the paste comprises a powder, such as a metallic or ceramic powder,
or a combination of both, an organic binder, optionally a rheology
modifier and optionally an anorganic binder, such as a colloidal
binder. By computer controlled movement in x, y and z-direction, a
porous architecture is built layer-by-layer. The x and y directions
typically refer to the plane of the layers 151-154, whereas the
z-direction is the direction of stacking of the layers
(perpendicular on the plane of the layers). This process can
involve multiple nozzles or a single nozzle. The green part which
is obtained by the above process can be post-processed in one or
two steps: an optional drying step followed by sintering. Sintering
may be carried out under vacuum conditions, or in an inert or
reducing atmosphere, e.g. to avoid oxidation in case of metals.
After sintering, a highly reproducible and periodic porous
structure is obtained. The process variables include the nozzle
opening (fibre thickness or diameter), the type of nozzle (fibre
shape), the inter-fibre distance (pore size) and the stacking of
the layers (architecture). The microporosity and surface roughness
of the fibres can be controlled. An equipment for 3DFD typically
comprises a paste reservoir with nozzle, mounted on an apparatus
with numerical control of three or more axes, e.g. an XYZ-table or
a CNC machine. Multiple nozzles can be mounted onto the equipment
to speed up the production of similar pieces.
Fibres 13, 14 of consecutive layers advantageously extend along
transverse directions and the fibres within the same layer are
advantageously spaced apart. By way of example, referring to FIG.
1, fibres 13 of layer 151 are parallel to one another and have
longitudinal axes cross to the longitudinal axes of fibres 14 in
the below layer 152. Fibres 13 and 14 may extend perpendicular to
one another, or oblique, e.g. at an angle different from 0.degree.
and different from 90.degree.. As a result, a highly porous
structure can be obtained. The fibres are advantageously, though
not necessarily arranged in an orderly fashion. By way of example,
fibres 13 within the same layer can be parallel, extend radially
from a common centre, be concentric in circles or extend
spirally.
To account for the sometimes complex geometry of surgical implants,
the scaffold structures 11 can be made as a block, e.g. by 3DFD as
described above, and machined afterwards, e.g. milled, to the
correct geometry, e.g. to fit on the dense part 12. The attachment
with the dense part 12 can be provided by known techniques, such as
sintering, friction welding, laser welding, etc.
Advantageous porous structures 11 may comprise longitudinal
channels extending substantially normal to the external surface
111, e.g. the longitudinal channels may extend from the surface 111
in a direction of approach of the interface 115 or the dense part
12. These longitudinal channels may be straight or tortuous. The
tortuosity may be defined by staggering the fibres as will be
described further below.
According to an aspect, a first porosity gradient may be provided
between the first regions 112 and the second regions 113. That is,
along a first direction, referred to as gradient direction, the
porosity, and therefore also the density of the structure 11, is
made to change. The first gradient direction advantageously lies on
the surface 111, or may be a direction at least locally tangential
to the surface 111.
By way of example, a first region 112 is provided with a porosity
P1. A possible adjacent second region 113 is provided with porosity
P3, which is different from P1, e.g. P1>P3. Possibly, an
intermediate region (not shown) may be interposed between first
region 112 and second region 113, which may be provided with
porosity P2, with P2 different from P1 and P3. According to an
aspect, the porosity changes along the first gradient direction
from a higher porosity P1 and hence lower density of the structure
11 in the first region 112 to a lower porosity P3 and hence a
higher density of structure 11 in the second region 113.
Advantageously, the porosity gradient is one with a porosity
decreasing from the first region, possibly through the intermediate
region, towards the second region. In other words,
P1>P2>P3.
According to yet another aspect, a second porosity gradient may be
provided in a direction substantially orthogonal to the first
gradient direction, e.g. a direction oriented away from or in
approach of the surface 111.
The local porosity can be determined based on the geometry of a
unit pore cell 20 as shown and defined in FIG. 2. A pore can be
regarded as a cell delimited on all sides by fibres 13, 14. The
stacking factor c refers to the interpenetration depth between
fibres of consecutive layers. The stacking factor is obtained, e.g.
during build of a 3DFD structure but is analogous with other
additive manufacturing processes, by increasing the (vertical)
build height (z) by an amount less than the fibre diameter, when
starting a new layer on top of the previous one. The fibre diameter
can be determined by optical microscopy or Scanning Electron
Microscope imaging of a cross-section of the material and is mainly
determined by the nozzle diameter of the 3DFD apparatus, printing
conditions and the shrinkage upon sintering. The stacking factor c
may be influenced by the paste composition (e.g. viscosity), fibre
thickness, inter-fibre distance and printing conditions such as
temperature and humidity. The stacking factor has a strong
influence on the mechanical strength of the fibres, but also
influences the macroporosity and the interconnectivity of the
macropores. The stacking factor c can be measured by means of a
Scanning Electron Microscope imaging. Further, a=M-n is fibre
diameter (mm), n is inter-fibre distance (mm) and M is axial centre
spacing between two fibres (mm). The macroporosity (P, %) of the
cell can be calculated as follows, with SSA being the specific
surface area (SSA, mm.sup.2/mm.sup.3), S.sub.c is the loss of the
surface area of two connected fibres (mm.sup.2), S.sub.f is surface
area of the two fibres (mm.sup.2), V.sub.cell is the unit cell
volume (mm.sup.3) and V.sub.fibre is the fibre volume
(mm.sup.3):
.times..times..times..times..times..times..times..pi..times..times..times-
..times..times..times..pi..times..times..function..times..times..times..ti-
mes..function..times..times..pi..times..times..times..times..times..times.-
.times..times..times..times..times..times..pi..times..times..function..tim-
es..times..times..times..function..times..times..times..times..times..time-
s..times..times..times. ##EQU00001## with V.sub.c the volume of the
intersection of two fibres with same fibre diameters a.
V.sub.c depends on the stacking factor c. The stacking factor c can
be in the range 0.ltoreq.c.ltoreq.a. While c=a, V.sub.c is a
"Steinmetz solid". Therefore,
.times. ##EQU00002## While c is 0<c<a, a circular cone volume
can be assumed for simplifying the calculation of V.sub.c, which is
an approximation of the real elliptic cone volume. Assuming a
circular cone volume:
.times. ##EQU00003## .times..pi..function..times.
##EQU00003.2##
Reference to porosity in the present description relates to
macroporosity, e.g. porosity between the fibres disregarding
porosity of or within the fibres. Advantageously, macropores have a
pore size of at least 10 .mu.m in diameter, advantageously a pore
size of at least 25 .mu.m, advantageously at least 50 .mu.m.
Absolute (macro)porosity values in structures according to aspects
of the disclosure are not particularly limiting. Advantageous
values are between 40% and 95% porosity, advantageously between 50%
and 80%. Average (macro)porosity values of porous structures
according to present aspects are advantageously between 50% and
90%, advantageously between 55% and 85%, advantageously between 60%
and 80%.
According to aspects of the disclosure, the difference (i.e. the
change) in porosity (expressed as a percentage) between the first
regions and the second regions is at least 4%, advantageously at
least 5%, advantageously at least 6%, advantageously at least 8%,
advantageously at least 10%. In other words, assuming the (volume)
porosity is P1(%) in the first region (evaluated at the surface
111), and P2(%) in the second region (evaluated at the surface
111), the difference in porosity .DELTA.P (%)=P1-P2. The porosity
may change between a porosity between 50% and 95%, advantageously
between 60% and 90%, advantageously between 70% and 90% in the
first region and a porosity between 40% and 80%, advantageously
between 50% and 70%, advantageously between 50% and 60% in the
second region.
In porous (scaffold) structures according to aspects of the
disclosure, the fibres advantageously have a diameter a in the
range between 20 .mu.m and 2 mm, advantageously between 40 .mu.m
and 1 mm, advantageously between 60 .mu.m and 600 .mu.m, with
advantageous values being 80 .mu.m, 100 .mu.m, 150 .mu.m, 200
.mu.m, 300 .mu.m, 400 .mu.m, 500 .mu.m. All fibres within a same
layer of the structure typically have a same diameter, and the
fibre diameter may be the same in all layers of the structure or
may change between layers, e.g. by using different nozzles with
different diameters for extruding the fibres.
The inter fibre distance n, e.g. within a same layer, may vary
between 0 .mu.m and 5 mm, and is advantageously between 10 .mu.m
and 2 mm, advantageously between 25 .mu.m and 1 mm, advantageously
between 50 .mu.m and 900 .mu.m, advantageously between 100 .mu.m
and 800 .mu.m, and advantageously at least 200 .mu.m,
advantageously at least 300 .mu.m. The inter fibre distance n
typically changes within one layer so as to obtain a change in
porosity, and advantageously to obtain a porosity gradient. In the
scaffold structures described herein, the interfibre distance
relates to the size of a pore cell 20.
The stacking factor c may vary between 0 and the fibre diameter a,
advantageously 0.01a.ltoreq.c.ltoreq.0.99a, advantageously
0.02a.ltoreq.c.ltoreq.0.90a, advantageously
0.03a.ltoreq.c.ltoreq.0.50a, advantageously
0.05a.ltoreq.c.ltoreq.0.20a. Advantageously, the ratio c/a is at
least 0.075, at least 0.1, at least 0.125, at least 0.15. The
stacking factor typically is constant within one layer, and may
change between layers. Typical values of the stacking factor c may
range between 10 .mu.m and 200 .mu.m, advantageously between 20
.mu.m and 150 .mu.m, advantageously between 30 .mu.m and 100 .mu.m,
e.g. 70 .mu.m.
Referring to FIG. 2, the fibre diameter a and the stacking factor c
define the size of the interconnection between adjacent pores
within a same layer, also referred to as pore throat 21. The size
of the pore throat 21, which may be defined as a-2c is
advantageously at least 20 .mu.m, advantageously at least 50 .mu.m.
The inter-fibre distance n and disposition of the fibres (e.g.
staggered or aligned) mainly defines the size of the
interconnection 22 between pores of consecutive layers. The pore
interconnections 22 define paths in a direction perpendicular to
the plane of the fibre layers, and therefore may define the pore
interconnection in a direction of depth of the structure 11,
starting from the external surface 111. The size of the pore
interconnections 21 and/or 22 relates to the pore interconnectivity
and may be important for the first regions with higher porosity to
promote bone ingrowth, or for the second regions with smaller
porosity to promote vascularisation, and may be important for both
regions. Typically, the size of the pore interconnections 22, may
be larger for the first regions and smaller for the second regions.
Possibly, the first regions and the second regions may have
different pore interconnection sizes.
Additive manufacturing techniques allow for easily and effectively
making monolithic structures with desired porosity gradients. For
porous structures built up out of an arrangement of fibres, the
easiest way of obtaining a porosity gradient is through changing
the spacing between (parallel) fibres within some or all layers,
i.e. the inter fibre distance n. One example is shown in FIG. 1
showing the disposition of the fibres 13, 14 as seen from a
direction orthogonal to the planes of the (parallel) layers. In
FIG. 1, the fibres 13 within a same layer are disposed parallel to
one another and the fibres 13 and 14 of consecutive layers are
transverse, e.g. orthogonal to one another. It can be observed that
in the first regions 112, the spacing (interfibre distance n.sub.1)
between adjacent fibres is larger as compared to the interfibre
distance (n.sub.2) of the structure in the second regions 113. This
change in the interfibre distance (or, in more general terms, the
spacing between adjacent fibres) can be applied to all layers, or
alternatively to some but not all the layers, e.g. only layers
having fibres parallel to fibres 13, to obtain a porosity change or
gradient.
In addition, or alternatively to a porosity difference on the
surface 111, the first regions and the second regions may have
different pore interconnectivity between consecutive layers, as
shown in FIG. 3. The scaffold structure 31 of FIG. 3 comprises
first regions 312 and second regions 313 having different pore
interconnectivity in the direction 16 perpendicular to the fibre
layers. This may be obtained by staggering the fibres 13 and 13' in
different layers. Even though in this example the size of a unit
pore cell remains the same between the first region 312 and the
second region 313 due to identical interfibre distance, the pore
cells in second regions 313 are staggered, which reduces the pore
interconnectivity between adjacent layers. It will be convenient to
note that a change in pore interconnectivity such as shown in the
example of FIG. 3 can be combined with the change in porosity such
as shown in the example of FIG. 1.
Referring to FIG. 4, a scaffold structure 41 is shown which differs
from the structure 11 of FIG. 1 in that in the first regions 412
and in the second regions 413 a porosity gradient is applied in the
structure 41 along a direction 16 of approach to, or away from, the
surface 111, i.e. a direction of approach to the dense part 12.
This porosity gradient is applied in addition to the change in
porosity between first regions 412 and second regions 413. In the
example shown, the porosity gradient is obtained by changing the
interfibre distance n, e.g. increasing n towards the external
surface 111 to obtain a higher porosity at or near the surface 111
and a lower porosity towards the dense part 12. It will be
convenient to note that such a porosity gradient can be applied to
only one of the first regions 412 and the second regions 413, or
alternatively to both.
Referring to FIG. 5, a scaffold structure 51 is shown which differs
from the structure 41 of FIG. 4 in that the porosity gradient along
direction 16 is (further) obtained by changing the stacking factor
c through the layers in the stack of structure 51. In the example,
c is increased from the external surface 111 towards the dense part
12. Changing the porosity by changing the stacking factor can be
applied in addition to, or in the alternative of changing the
interfibre distance.
Referring to FIG. 6, a scaffold structure 61 is shown which differs
from the structure 41 of FIG. 4 in that the porosity gradient along
direction 16 is obtained by a change of the fibre diameter through
different layers. Layers proximal to the external surface 111 may
comprise fibres 63 having a larger diameter compared to fibres 64
arranged in layers remote from the surface 111. Alternatively,
layers proximal to surface 111 may comprise fibres having a smaller
diameter compared to fibres of layers remote from surface 111. In
the latter case, a porosity gradient may be obtained by appropriate
selection of the inter fibre distance n in each layer, e.g. with n
decreasing in the direction 16. The change of fibre diameter as
described hereinabove may be combined with other ways of obtaining
a porosity gradient in direction 16, such as those described in
relation to FIG. 4 or 5.
It will be convenient to note that the fibres themselves may
comprise a microporosity, e.g. porosity with pore size smaller than
the size of the macropores as indicated above, as shown in FIG. 7.
The scaffold structure 71 of FIG. 7 differs from the structure 41
of FIG. 4 in that the fibres comprise a microporosity. The
microporosity may extend in a peripheral region, e.g. sheath 72, of
the fibres 73. In this case, the fibres 73 may comprise a dense
core 74. Alternatively, fibres 73 may be microporous
throughout.
Microporous fibres may be obtained by subjecting the fibres to a
phase inversion process as e.g. described in WO 2009/027525, 5 Mar.
2009, which is incorporated herein by reference. Biomedical
implants with macro- and microporous structure may stimulate
osseointegration and provide sufficient local mechanical strength
for fixation/implantation. Due to the macroporosity, the implant
materials can be easily coated with conventional coating procedures
such as dip-coating or wash-coating, with growth factors. Due to
the microporosity, the as such deposited coatings will have a much
better adhesion. Advantageously, the (microporous) fibres are
otherwise solid fibres, i.e. they are advantageously not
hollow.
The microporous fibre or filament morphology may be induced by
phase inversion. A method for producing such morphology may
comprise the steps of: a) preparing a suspension comprising
particles of a predetermined material, a liquid solvent, one or
more binders and optionally one or more dispersants, b) depositing
said suspension in the form of fibres or filaments in a layered
fashion, e.g. according to a predetermined disposition of fibres or
filaments, thereby creating a porous structure, c) inducing phase
inversion, whereby said filaments are transformed from a liquid to
a solid state, by exposing said filaments during the deposition of
the filaments to a non-solvent vapour and to a liquid non-solvent,
d) thermally treating the structure of step c) by calcining and
sintering said structure. In other words, step c) of the present
method involves the step of exposing the filaments during the
deposition of the filaments to a non-solvent vapour and to a liquid
non-solvent, such that the deposited filaments solidify and obtain
surface roughness and microporosity. In a preferred embodiment,
step b) is carried out in a non-solvent environment.
Advantageously, an alternative step c) comprises the step of c1)
bringing the filaments during the deposition of the filaments into
contact with a non-solvent vapour, and the step of c2) immersing
the structure of step c1) in a liquid non-solvent, thereby creating
a filament-based porous structure having suitable filament
morphology. Phase inversion can be completed in a next step (step
c2) of the present method by immersing the structure in a liquid
non-solvent.
The fibres or filaments in the sintered porous structure obtained
after step d) advantageously comprise an average surface roughness
(Ra) which is higher than 4 .mu.m. Moreover, the filaments in the
sintered porous structure obtained after step d) also have a
microporosity (after sintering) comprised between 1 and 50%,
preferably between 5 and 30%. Microporosity refers to a porosity
wherein the pores have a size smaller than macropores as indicated
above.
Referring to FIG. 8, a possible arrangement of fibres is shown.
This arrangement is particularly suited as scaffold structure for
the acetabular component indicated above. The structure 81
comprises a repeating pattern of three consecutive layers. In a
first layer, fibres 83 are arranged in concentric circles. A second
layer comprises fibres 84 arranged parallel to one another. A third
layer comprises fibres 85 arranged parallel to one another. Fibres
85 are perpendicular to the fibres 84 of the second layer. The
porosity difference between different regions or areas in structure
81 can be brought about by changing the inter-fibre distance in
one, some or all three layers. As shown in FIG. 8, the in the fibre
distance is changed in each one of the three layers in a graded way
to obtain different regions with different porosity. It will be
convenient to note that the first, second and third layer may be
arranged in any order.
The materials of which the porous structures according to aspects
of the present disclosure are made include metals, ceramics, and
composite materials, in particular those materials being
biocompatible.
The present disclosure may include one or more of the following
concepts: A. A surgical implant (10), comprising a porous structure
(11) with interconnected pores (20) for ingrowth of bone into the
porous structure, wherein the porous structure comprises an
arrangement of fibres (13, 14) which are attached to one another,
wherein the fibres are arranged in layers (151, 152, 153), the
layers being stacked, characterised in that the porous structure
comprises a surface (111), wherein the surface comprises different
regions (112, 113) having different porosities, the different
porosities being determined by the arrangement of fibres. B. The
surgical implant of paragraph A, wherein the porous structure (11)
is attached to a dense part (12), and wherein the surface is
located opposite the dense part. C. The surgical implant of
paragraph B, wherein the dense part comprises an interface (115)
with the porous structure, wherein a direction (16) of stacking of
the layers of the porous structure is oriented in a direction of
approach to, or away from the interface. D. The surgical implant of
any one of the preceding paragraphs, wherein the surface comprises
a first porosity in a first region (112) on the surface and a
second porosity in a second region (113) on the surface, the
difference between the first porosity and the second porosity being
at least 4%. E. The surgical implant of paragraph D, wherein the
difference between the first porosity and the second porosity is at
least 6%. F. The surgical implant of paragraph D or E, wherein the
first porosity is between 45% and 90% and wherein the second
porosity is between 40% and 85%. G. The surgical implant of
paragraph F, wherein the first porosity is between 70% and 90% and
wherein the second porosity is between 40% and 60%. H. The surgical
implant of any one of the preceding paragraphs, wherein the porous
structure has an average porosity between 50% and 80%. J. The
surgical implant of any one of the preceding paragraphs, wherein
the fibres (13, 14) have a diameter between 20 .mu.m and 5 mm. K.
The surgical implant of any one of the preceding paragraphs,
wherein fibres of consecutive layers interpenetrate, wherein a
ratio between a penetration depth (c) between the fibres of the
consecutive layers and a diameter (a) of the fibres is between 0.05
and 0.5. L. The surgical implant of paragraph K, wherein the ratio
is between 0.1 and 0.5. M. The surgical implant of any one of the
preceding paragraphs, wherein a spacing (n) between adjacent fibres
of a same layer is between 10 .mu.m and 5 mm. N. The surgical
implant of paragraph M, wherein the spacing (n) between the fibres
in at least one layer changes between the first region and the
second region to obtain the different porosities. O. The surgical
implant of any one of the preceding paragraphs, wherein the porous
structure comprises a porosity gradient in a direction (16)
orthogonal to the surface (111). P. The surgical implant of any one
of the preceding paragraphs, wherein the porous structure (11) is
attached to a dense part (12), wherein the surface is located
opposite an interface (115) of the porous structure with the dense
part, and wherein the porous structure comprises a porosity
gradient wherein the porosity decreases in a direction (16) of
approach to the dense part. Q. The surgical implant of paragraph O
or P, wherein a penetration depth (c) between the fibres of
consecutive layers changes along a direction of the porosity
gradient. R. The surgical implant of any one of the preceding
paragraphs, wherein the fibres comprise micropores. S. The surgical
implant of any one of the preceding paragraphs, wherein the
different regions (112, 113) having different porosities alternate
on the surface. T. A method of making a surgical implant (10),
comprising forming fibres (13, 14) in layers, the layers being
stacked; connecting the fibres of consecutive layers to one another
to obtain a porous structure (11); characterised in that the method
comprises the step of arranging the fibres in proximity of a
surface (111) of the porous structure with different interspaces in
different regions of the surface such that the different regions
(112, 113) have different porosities. U. The method of paragraph T,
comprising making a dense part and attaching the porous structure
to the dense part.
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